A primary goal in rechargeable battery research is developing batteries with higher specific energies, with motivations including increasing electric vehicle range and enabling new deep space technologies. One such option, the nonaqueous lithium-oxygen (Li-O2) battery, consists of a lithium negative electrode, a lithium salt and ether-based electrolyte, an electrolyte-soaked porous carbon positive electrode, and a gaseous oxygen headspace, and operates via the electrochemical formation (discharge) and decomposition (charge) of lithium peroxide (Li2O2). With an estimated theoretical specific energy of 3330 Wh/kg active material (Li2O2), more than four times that of current lithium-ion positive electrode materials, and a relatively low cost of battery components, the nonaqueous lithium-oxygen (Li-O2) battery has garnered significant research attention over the past decade.
Unfortunately, critical challenges have been identified that prevent the realization of a high-capacity, rechargeable Li-O2 battery. The ultimate discharge product, Li2O2, is insoluble in the most stable nonaqueous electrolytes and is a wide-band gap insulator, so during discharge it forms as a solid on the cathode’s carbon support and electronically passivates it, preventing further discharge after only a small amount of Li2O2 has formed. Li2O2 and its electrochemical intermediates also undergo irreversible side reactions with the nonaqueous electrolytes and carbon positive electrodes studied to-date, causing poor battery rechargeability.
In this work, the nonaqueous electrolyte of the Li-O2 was engineered toward addressing these challenges and achieving a high-capacity, rechargeable Li-O2 battery. Toward increasing achievable discharge capacity, the ability of electrolytes to induce solubility of the intermediate to Li2O2 formation, lithium superoxide (LiO2), was studied, as this enables a solution mechanism of growth whereby Li2O2 grows in large, aggregated structures, allowing more Li2O2 to form before cathode passivation. First, the effect of the lithium salt anion on LiO2 solubility was studied. To do so, a typical lithium battery salt, lithium bis(trifluoromethane) sulfonimide (LiTFSI), was partially exchanged for the more strongly electron-donating lithium nitrate (LiNO3) in Li-O2 battery electrolytes. During galvanostatic conditions, a correlation between LiNO3 concentration and discharge capacity was observed. Titrations and scanning electron microscopy of cathodes extracted from discharged batteries confirmed Li2O2 formation in aggregated structures in cells that partially employed LiNO3 as an electrolyte, indicative of an increase in the solution mechanism with the addition of LiNO3. The increase in LiO2 solubility was attributed via 7Li NMR to a lower free energy of Li+ in the electrolyte as a result of the addition of the strongly electron donating NO3- in the lithium solvation shell. Differential electrochemical mass spectrometry (DEMS) showed similar oxygen evolution on charge with and without LiNO3, indicating no deleterious effect on cell rechargeability with the addition of LiNO3.
Second, as anion selection induces the solution mechanism by lowering the free energy of Li+ in solution, non-Li alkali metal cations and alcohols were studied as methods of inducing the solution mechanism by lowering the free energy of the superoxide anion (O2-) in solution. Galvanostatic cycling of Li-O2 batteries containing non-Li alkali metal salts showed a small increase in the achievable discharge capacity, attributed to the softer acidity of non-Li alkali metal cations more favorably solvating the soft base O2-. However, gas analysis of a sodium-oxygen battery with a small amount of Li+ salt added to the battery electrolyte showed Li+ quickly scavenges any non-Li alkali metal cation-associated O2-, and the resultant LiO2 quickly disproportionates into the insoluble Li2O2. It is therefore anticipated that an increase in Li-O2 battery capacity upon the addition of non-Li alkali metal cations is only expected at high currents, when oxygen reduction rates are sufficiently high to allow some O2- to temporarily avoid Li+ in solution. Ppm quantities of methanol, ethanol, and 1-propanol were added to ether-based Li-O2 battery electrolytes as analogues to water, which has been previously shown to induce the solution mechanism due to its strong Lewis acidity lowering the free energy of O2- in solution. The additives induced a two-fold increase in battery capacity, though with little trend in the capacity as a function of the additive’s Acceptor Number. These results highlight the complexity of interactions between the constituent species in an electrolyte in terms of their Lewis basicities, Lewis acidities, and other physicochemical properties.
While the formation of Li2O2 in large aggregated structures increases the achievable discharge capacity, an electrolyte-soluble redox mediator is required to oxidize aggregated Li2O2 on charge and shuttle electrons back to the electrode surface. The rechargeability of Li-O2 batteries containing redox mediators in the presence of water impurities, which are likely difficult to eliminate in practical lithium-air batteries, was studied. Specifically, the effect of water contamination in the electrolyte on the promising redox mediator lithium iodide (LiI) was studied. DEMS and titrations of cathodes extracted from discharged batteries confirmed recent reports that lithium hydroxide (LiOH) formed as the dominant discharge product via a 4 e-/O2 process. However, isotopic labeling and DEMS were used to show LiOH is not reversibly oxidized back to its reactants (Li+, O2, H2O). Rather, titrations, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), and galvanostatic cycling of batteries under an argon atmosphere showed charge current in batteries containing both LiI and H2O is a complex mixture of side reactions and redox shuttling.
With LiOH identified as an undesirable discharge product, the mechanism for Li2O2 degradation to LiOH in the presence of LiI and H2O was studied. Galvanostatic cycling of lab-scale Li-O2 batteries containing LiI and H2O in DME and dimethyl sulfoxide (DMSO) showed that DMSO prevents Li2O2 degradation to LiOH. Cyclic voltammetry of these electrolytes showed DMSO exhibits a higher potential for iodide oxidation than DME, indicating iodide-mediated H2O2 reduction is more difficult in DMSO than DME. The ability of an additive to reduce H2O2 is therefore identified as a key consideration in Li2O2’s stability in the presence of water impurities. A tangential important finding during this study was the difficulty in selection of an appropriate reference electrode for studying redox mediators in Li-O2 batteries, as electrodes with too high of a lithium intercalation voltage will chemically oxidize the redox mediator, while electrodes with too low of a lithium intercalation voltage exhibit spontaneous oxygen consumption in a Li-O2 battery.